The present invention relates to a method and system for diagnosing improving the vision of an eye.
Most common defects in human vision are caused by the inability of the eye to focus properly. For example, nearsightedness can be attributed to an eye which focuses forward of the retina instead of on it, farsightedness can be attributed to an eye which focuses beyond the retina, and astigmatism can be attributed to an eye which cannot produce a sharp focus, instead producing an area of blurriness. Ophthalmologists model the cornea as a portion of an ellipsoid defined by orthogonal major and minor axes. Current surgical procedures for correcting visual acuity are typically directed at increasing or decreasing the surface curvature of the cornea, while making its shape more spherical.
In conjunction with modern corneal procedures, such as corneal ablation surgery, and for clinical applications, high resolution cameras are used to obtain a digitized array of discrete data points on the corneal surface. One system and camera useful for mapping the cornea is the PAR Corneal Topography System (PAR CTS) available from PAR Vision Systems. The PAR CTS maps the corneal surface topology in two-dimensional Cartesian space, i.e., along x- and y-coordinates, and locates the xe2x80x9cline-of-sightxe2x80x9d, which is then used by the practitioner to plan the surgical procedure. The xe2x80x9cline-of-sightxe2x80x9d is a straight line segment from a fixation point to the center of the entrance pupil. As described more fully in Mandell, xe2x80x9cLocating the Corneal Sighting Center From Videokeratography,xe2x80x9d J. Refractive Surgery, V. 11, pp. 253-259 (July/August 1995), a light ray which is directed toward a point on the entrance pupil from a point of fixation will be refracted by the cornea and aqueous and pass through a corresponding point on the real pupil to eventually reach the retina.
The point on the cornea at which the line-of-sight intersects the corneal surface is the xe2x80x9coptical centerxe2x80x9d or xe2x80x9csighting centerxe2x80x9d of the cornea. It is the primary reference point for refractive surgery in that it usually represents the center of the area to be ablated in photorefractive keratectomy. The line-of-sight has conventionally been programmed into a laser control system to govern corneal ablation surgery. However, some surgeons prefer to use the pupillary axis as a reference line. Experienced practitioners have employed various techniques for locating the sighting center. In one technique, the angle lambda is used to calculate the position of the sighting center relative to the pupillary (xe2x80x9copticxe2x80x9d) axis. See Mandell, supra, which includes a detailed discussion of the angles kappa and lambda, the disclosure of which is incorporated herein by reference as if set forth in its entirety herein.
In current corneal ablation procedures, a portion of the corneal surface is ablated. The gathered elevational data is used to direct an ablation device such as a laser so that the corneal surface can be selectively ablated to more closely approximate a spherical surface of appropriate radius about the line-of-sight, within the ablation zone. The use of the line-of-sight as a reference line for the procedures may reduce myopia or otherwise correct a pre-surgical dysfunction. However, a more irregularly shaped cornea may result, which may exacerbate existing astigmatism or introduce astigmatism in the treated eye. This will complicate any subsequent vision correction measures that need be taken. Also, any substantial surface irregularities which are produced can cause development of scar tissue or the local accumulation of tear deposits, either of which can adversely affect vision.
Implicit in the use of the-line-of sight or the pupillary axis as a reference axis for surgical procedures is the assumption that the cornea is symmetric about an axis extending along a radius of the eye. The cornea, however, is an xe2x80x9casymmetrically asphericxe2x80x9d surface. xe2x80x9cAsphericxe2x80x9d means that the radius of curvature along any corneal xe2x80x9cmeridianxe2x80x9d is not a constant (a xe2x80x9cmeridianxe2x80x9d could be thought of as the curve formed by the intersection of the corneal surface and a plane containing the pupillary axis). Indeed, the corneal curvature tends to flatten progressively from the geometric center to the periphery. xe2x80x9cAsymmetricxe2x80x9d means that the corneal meridians do not exhibit symmetry about their centers. The degree to which the cornea is aspheric and/or asymmetrical varies from patient to patient and from eye to eye, within the same person.
Clinical measurements performed with the PAR CTS, as analyzed in accordance with the method disclosed in U.S. Pat. No. 5,807,381 assigned to the assignee of the present patent application, reveal that the cornea exhibits a tilt, typically a forward and downward tilt, relative to the eye. This tilt may be as great as 6xc2x0 and, on the average, is between 1xc2x0 and 3xc2x0. Hence, a corneal ablation procedure which utilizes the line-of-sight or pupillary axis as a reference axis tends to over-ablate some portions of the cornea and underablate other portions of the cornea. At the same time, it changes the geometric relationship between the cornea and the remainder of the eye. Thus, any ablation procedure which does not take into account the tilt of the cornea is not likely to achieve the desired shaping of the cornea and may therefore be unpredictable in its effect.
Analysis of clinical measurements in accordance with the method of U.S. Pat. No. 5,807,381 also reveals that the point on the surface of the cornea which is most distant from the reference plane of the PAR CTS (hereafter referred to as the HIGH point) is a far more effective reference point for corneal ablation than the center of the cornea. Specifically, as demonstrated in U.S. Pat. No. 5,807,381 laser ablation about an axis passing through the HIGH point produces a much more regularly shaped cornea and removes substantially less corneal material than the same operation performed about an axis close to the center of the eye, such as the pupillary axis.
Although incorporating corneal tilt and utilizing the HIGH point leads to improved and more consistent results with corneal ablation surgery, there is still an excessively high degree of unpredictability. For example, recent analyses of clinical measurements have revealed that the post-operative cornea begins to change shape a short time after corneal ablation surgery. Thus, a nearly perfectly spherical post-operative cornea, will, over time, return to an aspheric, asymmetric shape.
The use of a collagen gel has been proposed as a vehicle to facilitate smoothing of the corneal undulations. See Ophthalmology Times, xe2x80x9cSlick Start, Clear Finish,xe2x80x9d 1995, pp. 1 and 24 (Jun. 19-25, 1995) and Review of Ophthalmology, xe2x80x9cNews and Trends: Researchers Unveil New Ablatable Mask,xe2x80x9d pp. 12-13 (June 1995), the disclosures of which are incorporated herein by reference as if set forth in their entirety herein. A Type 1 collagen is molded between a contact lens and the anterior surface of the cornea to form a gel mask. The surgeon can adjust the curvature of the postoperative cornea by selecting a flatter or steeper lens, as desired. Reportedly, the gel mask does not shift when hit by laser pulses. Therefore, instead of selective ablation of predetermined locations of the cornea, the masked cornea can be ablated to a uniform depth, thereby conforming the surface contour of the cornea to the lens. A smooth post-operative cornea results, and refractive power correction can be achieved. However, because the ablation operation is centered on the optical center of the cornea or the center of the pupil and does not allow for corneal tilt, the postoperative eye may exhibit an irregular shape or more corneal material may be removed than is necessary.
What is needed in the art and has heretofore not been provided is a method of correcting vision that avoids one or more of these problems, that can produce predictable results, and that provides corrected vision with respect to the particular topology of the patient""s eye on which the correction is being performed.
It is therefore one object of the present invention to provide a method for improving the vision of an eye.
It is an additional object of the present invention to provide an improved surgical method for a corneal ablation procedure.
It is also an object of the present invention to provide a method and apparatus for diagnosing and analyzing a pre-surgical eye for the purpose of predicting the post-operative condition of the eye and planning more effective surgery.
The present inventors believe that corneal ablation surgery has had limited success and predictability, because of a parochial approach. The conventional wisdom has been to concentrate on the shape of the cornea, with the expectation that a smooth, spherical cornea will optimize vision. However, the human eye is a complex system which includes numerous optical components besides the anterior surface of the cornea (for example, the posterior corneal surface, the lens and the aqueous), all of which affect vision. Also, the mechanical environment of the eye cannot be ignored. For example, recent analyses of clinical measurements reveal that the eyelids exert substantial pressure on the cornea, causing it to flatten near its upper margin and to form a depression near its lower margin. It is believed that the mechanical environment of the eye accounts, in large part, for its shape. This also explains why a perfectly spherical post-operative cornea would return to an aspherical, asymmetric shape.
In accordance with the present invention, corneal ablation procedures of the eye are performed in a manner which does not interfere with the natural shape of the cornea or its orientation relative to the remainder of the eye, but which changes its surface curvature appropriately to achieve the required correction of vision. Three preferred embodiments are described, which model the cornea to different degrees of accuracy. Once the model of the cornea is obtained, surface curvature is modified to achieve the degree of correction in refraction that is necessary, as determined by an eye test of the patient. The modified model of the cornea is then utilized to control the removal of material from the surface of the cornea in a corneal ablation operation.
In a first embodiment, the cornea is modeled as an ellipsoid having major and minor axes which are perpendicular to each other. These are the axes that are revealed by conventional eye tests as being appropriate for correction of refraction. On a model of the cornea generated in accordance with disclosure of U.S. Pat. No. 5,807,381 perpendicular planes are constructed which contain the local or tilted Z axis and are rotated about that axis to the angle specified by the eye test. The intersection of each of these planes with the surface model produces an arcuate curve. Each of these curves is then estimated by a circular arc which estimates the patient""s current radius of curvature at each axis. A modified arc is then determined which achieves the required diopter correction at each axis. A model of the post-operative cornea is then created by performing a smooth interpolation from one of the arcs to the other. In this model, the corneal surface is represented as the surface of an ellipsoid which has the corrected radii of curvature at the two orthogonal axes specified by the eye test.
In an second embodiment, the cornea is modeled in such a manner as to preserve its asymmetry. To achieve this, a large number of annularly spaced meridians are generated on the surface model of the cornea. The distance along each meridian is measured from the HIGH point to the perimeter of the working area of the cornea, and the curves with the greatest and least average radius of curvature are each estimated by a circular arc. The complementary curves corresponding to the two initial curves (i.e. those extending from the HIGH point diametrically opposite to the corresponding curve are then also estimated by circular arcs. Each of the four arcs is then adjusted for curvature to achieve the desired degree of visual correction at each arc. The model of the post-operative cornea is then generated by angularly interpolating between pairs of the four arcs mentioned above and providing smoothing between two partial surfaces at each of the four initial arcs.
A third embodiment of the invention comes closest to preserving the initial shape of the cornea. Initially, a large number of angularly spaced meridians, for example 72, are generated on the surface model. The curves defining the meridians, which extend from the HIGH point to the periphery of the working region of the cornea are each estimated by a circular arc. Each of these arcs is then corrected in curvature to achieve the required diopter correction at the respective arc. The post-operative corneal surface is then estimated by generating a best-fit surface corresponding to all of the corrected arcs.